Ocean Climate

Ocean Climate

Ocean climate is the average over a long time of marine features such as temperature, salinity, nutrients, waves, stratification and winds. Data on ocean climate conditions — such as the average July and January ocean temperature over a recent 30-year period — are often reported in climate information.

Interactions between the oceans, sea ice, snow pack and the atmosphere are a fundamental part of the Earth's global climate system. Understanding the role of oceans in global climate and the impacts of climate change on aquatic ecosystems is of critical importance to the international community and countries such as Canada, which borders three interconnected oceans.

What is the North Atlantic Oscillation?

The North Atlantic Oscillation (NAO) is a large-scale variation in atmospheric pressure over the North Atlantic Ocean and a key indicator of climate conditions in the region. Large-scale spatial variations in ocean temperature and salinity in the North Atlantic are related to the NAO index, which represents the relative strengths of atmospheric pressures over Iceland and the Azores.

Ocean Climate in Canada's Oceans

Gulf of St. Lawrence:

In addition to fishing, changes in ocean climate also directly and indirectly contribute to changes in marine populations and communities. Over the past four decades, changes in the North Atlantic Oscillation (NAO) and other changes in large scale weather patterns that persist for several years or more have led to profound changes in water temperatures in the Gulf of St. Lawrence.

During the summer, the Gulf typically has three temperature layers: a warm, relatively fresh (less salty) surface layer; a cold intermediate layer from about 50 to 150 metres; and, a deeper, warm salty layer covering channels and other areas that are deeper than 200 metres. Observed temperature variations in the Gulf include:

  • Surface water temperatures during the ice-free months have warmed by 1.5°C between 1982 and 2010.
  • Temperatures in the cold intermediate layer shifted from exceptionally warm during the later 1960s and early 1980s, to very cold from 1986 to 1998. Recent conditions have been closer to average.
  • Exceptionally cold conditions occurred at depths of around 200 to 300 metres from 1991 to 1996. Although waters have since warmed and have recently been close to their long-term (1981 to 2010) average of 4.5°C at 200 metres and 5.5°C at 300 metres, conditions were colder than average again in 2009-2010 but returned to average in 2011.

Scotian Shelf:

Temperature and salinity on the Scotian Shelf have a vertical structure that varies seasonally, similar to that in the Gulf of St. Lawrence, with lighter "shelf" water overlying saltier "slope" water that intrudes from offshore at depth. This results in a vertical gradient in water density that is referred to as "stratification". In the near-surface waters, this stratification is greatest in summer due to surface heating and the inflow of fresh water from the Gulf of St. Lawrence, and weakest in winter when cool winds increase vertical mixing that breaks down the stratification and brings important nutrients towards the surface.

Records of temperature and salinity dating back to about 1920 indicate that the largest changes that lasted several years on the Scotian Shelf occurred in the 1960s, when the intruding slope water was cooler and fresher. This arose from the enhanced flow of subpolar slope water around the Grand Bank during a period of negative NAO index (reduced wind forcing over the northern North Atlantic). The long-term trends in temperature and salinity on the Scotian Shelf vary with location and depth, and are generally weak, in part because of the strong natural (e.g., NAO) climate variability in the region. However, there is an indication of surface warming at most locations, and of increasing upper-ocean stratification across the Scotian Shelf and Gulf of Maine that is associated with a varying combination of surface warming and freshening. These changes are in the directions expected from anthropogenic climate change, and thus point to the possible emergence of a biologically important longer-term trend.

Beaufort Sea:

In the Beaufort Sea LOMA, some key changes in the ecosystem structure are episodic while others are more persistent. An example of a persistent change is the freshening of surface waters in the Canada Basin, observed since 2003. More fresh water in the LOMA has affected ocean structure (i.e., stratification of water layers) and the delivery of nutrients required for phytoplankton growth.

An episodic upwelling event of unprecedented intensity and duration occurred between November 2007 and February 2008 on the Mackenzie shelf. The salinity of water near the ocean bottom is generally around 33 units. During the upwelling event, bottom water salinity exceeded 34.5 units at the middle and outer shelf and was even higher (35-36.5 units) on the inner shelf. This high salinity water persisted for about two months. The changes in water salinity — caused by both the upwelling of deep salty water from the Canada Basin and the release of brine that occurs during sea ice growth — led to changes in ecosystem structure including increases in the growth of algae on the sea ice and phytoplankton in the water column. These changes in ecosystem structure highlight the importance of interactions and the cumulative effects of different factors.

Placentia Bay-Grand Banks:

Variations in the North Atlantic Oscillation (NAO) index can affect ice flow, ocean temperature and the strength of the Labrador Current. A high NAO index generally indicates colder water temperatures, stronger northwest winds, cooler air temperatures, and heavy ice sea conditions in the Northwest Atlantic, which was the pattern for most of the 1980s and 1990s. In winter 2010, the NAO index hit a record low, weakening the outflow of Arctic air to the Northwest Atlantic. This led to broad-scale warming (relative to 2009) throughout the Northwest Atlantic from West Greenland to Baffin Island to Newfoundland.

Water temperatures have a very important influence on the distribution and biology of marine animals. Changing water temperatures in this LOMA over the past four decades are thought to be responsible for some of the major changes in distribution and abundance of important commercial species.

Ocean temperature observations in the Placentia Bay-Grand Banks large ocean management area include:

  • At Station 27, a monitoring site off Cape Spear, NL, the 2010 depth-averaged annual water temperature increased by 0.7°C, making it the second highest on record.
  • Annual surface and bottom temperatures at Station 27 were also about 0.6°C above normal.
  • The area of the cold-intermediate-layer that was less than 0°C declined on the eastern Newfoundland Shelf, which is indicative of warmer than normal water temperatures.
  • Spring bottom temperatures in NAFO Divisions 3Ps and 3LNO during 2010 were above normal, resulting in a reduced area of the bottom habitat that was covered by water colder than 0°C.

Pacific North Coast:

In recent decades there have been increasingly frequent shifts between warm El Niño conditions (2010) and cool La Niña winters (2011). The El Niño/La Niña-Southern Oscillation, or ENSO, is a climatic pattern that occurs across the tropical Pacific Ocean about three to five years, although it can occur more frequently. It involves variations in the surface temperature of the equatorial Pacific Ocean that are set up by variations in tropical Pacific air pressure, known as the Southern Oscillation.

In the winter of 2010, El Niño — a warming of the ocean surface along the Pacific Equator — brought warm winds from the southwest along the west coast of the U.S. and Canada, pushing warm waters toward the British Columbia coast. In 2008, 2009 and 2011, La Niña (the cold phase of mid-Pacific equatorial waters) brought cool westerly winds and cool ocean surface waters to British Columbia.

What are Copepods?

Copepods are a diverse group of aquatic crustaceans and an important part of the zooplankton community. These tiny organisms form a critical part in the marine food web, linking microscopic phytoplankton to juvenile fish such as cod. Adult copepods are usually 1 to 2 millimetres (mm) in length, although the adults of some species may be as short as 0.2 mm or as long as 10 mm.

Ocean waters in the region are generally becoming warmer and less saline:

  • Daily sampling of ocean temperature at lighthouses on Kains Island and Langara Island in PNCIMA reveals warming by 0.5 to 0.6°C over the past 80 years.
  • In 2010, an El Niño year, all stations reported an increase in temperature (compared to 2009, a La Niña year) ranging from 0.5 to 1°C.
  • The largest decline in salinity was observed at Langara Island, along the coast of British Columbia, beginning in the late 1970s and accelerating through the 1990s and 2000s. This change may be due to the expansion of the Aleutian Low during winter in the late 1970s. Since then, this low pressure system has generally remained larger, which could affect salinity at Langara Island by altering winter winds and wind-driven ocean currents in the Gulf of Alaska.

Impacts of Changing Ocean Climate

Ocean temperatures can affect the growth and survival of marine life and the availability of the preferred and tolerated thermal habitats for various species. Changes in climate may also affect stock productivity and the sustainable harvest rates. Fishing could also exacerbate the impacts of temperature changes by decreasing stock resilience or increasing the variability in abundance and, therefore, the risks of a stock collapse.

Gulf of St. Lawrence:

Changes in ocean temperature in the St. Lawrence Estuary and Gulf are expected to affect the habitat, distribution and recruitment of marine species as well as community composition.

Observed and projected warming trends for surface waters in the Gulf will likely reduce the available habitat for certain temperature-sensitive species that now inhabit areas of the coastal zone. For example, temperatures over 23.5°C are lethal to Giant Scallop (Placopecten magellanicus) as are sudden increases to temperatures of 20oC.  In contrast, the habitat of warmer water species such as lobster, which is currently limited to coastal waters in the Gulf, is likely to increase in area with projected warming.

Long-term changes in surface water temperature have also affected the timing, duration and intensity of plankton production, which impacts the recruitment (the annual rate at which new individuals increase the population) of key fisheries resources. For example, the recruitment success of Northern Shrimp in the northern Gulf is closely positively linked to spring oceanographic conditions such as the warming rate of the sea surface and the duration and productivity of the phytoplankton spring bloom. Similarly, the recruitment of Atlantic Mackerel is positively linked to the production of specific copepod species in the southern Gulf and, ultimately, to regional oceanographic conditions.

From 1986 to 1998, when the cold intermediate layer was exceptionally cold, there was an increase in species of Arctic and more northern origin in the southern Gulf including Polar Sculpin (Cottunculus microps), Arctic Sculpin (Myoxocephalus scorpioides) and Arctic Cod (Boreogadus saida). Their sudden appearance as waters cooled in the 1990s — and disappearance as they warmed — is consistent with a distributional shift.

Bottom temperatures also affect the distribution, and potentially the abundance, of several other species. Long-term changes in the thickness and core temperature of the cold intermediate layer affect the bottom temperature on the Magdalen Shallows of the southern Gulf. In some years, bottom waters colder than 0°C were non-existent by September, while in other years they covered as many as 25,000 square kilometres of the bottom. Snow Crab prefer cool waters in winter (-1 to 3°C). The cooling and expansion of the cold intermediate layer during the late 1980s to early 1990s may have led to the extended distribution of Snow Crab stock and contributed to high abundances during and following that period.  However, a conclusive link has yet to be made due to the complex relationship between Snow Crab distribution, productivity and temperature.

The complexity of the many variables at play makes it very difficult to project with certainty how global warming will alter marine species and communities in Gulf. We can anticipate that warming will likely reduce habitat for some species that now inhabit the southern Gulf (e.g., Snow Crab, Capelin) and that it will likely create new habitat for more southerly species. Some species may shift to deeper waters or move northward. Global warming is also expected to bring increased variability in climate, leading to variations in the recruitment, growth and mortality of species and, as a result, their abundance. Some of the most profound changes to the marine communities may result from indirect effects of warming, such as hard-to-predict changes to the food web structure.

Scotian Shelf:

There have been no significant ecological impacts on the Scotian Shelf due to climate change but impacts may increase slowly over time (e.g., decades) or as an ecosystem shift (e.g. increased subtropical influences) at some future time.  In the short term, changes in the timing of the strong seasonal cycle may have more impact than a slow increase in temperature. While a comprehensive and precise assessment is not yet possible, there is enough knowledge to broadly assess potential climate change impacts. Climate change affects species’ physiology, timing of seasonal events and distribution. Those changes will in turn affect interactions between species, which impacts the species composition of an ecosystem.

Lower levels of the marine food web such as phytoplankton are greatly influenced by climate variability. Changing oceanographic conditions affect both the abundance and composition of phytoplankton communities. In general, if surface layers continue to warm, we should expect smaller-sized phytoplankton. If much higher temperatures lead to smaller organisms, energy flow through the ecosystem would be re-directed or less efficient, and might not support the productivity of historical fisheries. If increased stratification persists, there could also be significant changes in the seasonal cycle of phytoplankton growth, in part due to a reduced supply of nutrients into the surface layer where phytoplankton grow.

There is no question that climate plays a critical role in fish dynamics on the Scotian Shelf, but so does fishing. Internationally, over the past few decades, researchers have tried to separate the effects of climate and fishing on ecosystems. Increasingly, there is acknowledgement that the effects of climate and exploitation cannot be separated. Heavy fishing causes a reduction in diversity from the individual to the ecosystem level and diversity is the main buffer against climate variability. Intense fishing can lead to a loss of older, larger organisms, loss of sub-populations and a change in life-history traits, all of which render them much more susceptible to climate variability and chance events.

Beaufort Sea:

An increase in fresh water into the Canada Basin since 2003 has increased stratification and reduced water column mixing and the movement of nutrients from deeper layers into the sun-lit surface layer.

This has led to an increase in the smallest algae (picoplankton) in the Canada Basin, both in total amount and as a percentage of the total phytoplankton, and a decrease in larger nanoplankton. These early responses provide an indication of the potential to alter other parts of the marine food web. Some small plankton responded differently in 2009, highlighting the need for a long time series of data to assess ecosystem responses.

As a result of the 2007-2008 upwelling event on the Mackenzie shelf in this LOMA, the production of ice algae, phytoplankton, zooplankton and bottom-dwelling organisms increased by two- to six-fold. There was an overall increase in biological productivity, providing an opportunity to thrive for consumers such as zooplankton, which can adapt to the rapid change in ecosystem structure.

Pacific North Coast:

Increasingly frequent shifts between warm El Niño conditions and cool La Niña winters influence ocean life. For example, the abundance of certain copepod groups is strongly linked to annual changes in water temperature and circulation. Boreal and sub-arctic copepods, which tend to be more nutritious than southern copepods, were most abundant in cool years such as the early 1980s, 1999-2002, and 2007-2009. This benefited the survival and growth of young salmon, Sablefish and planktivorous seabirds.

Addressing Ocean Climate

Ongoing monitoring is essential to determine the ecological responses and interaction to persistent and year-to-year changes in ocean climate, including the impacts on commercial species. This knowledge will aid in the development of sustainable and flexible fisheries management plans in the face of changing ocean climate conditions. Phytoplankton, which forms the foundation of the marine food web, should be intensively monitored as “sentinels of climate change.” Changes in species composition are currently under investigation. 

From a global perspective, the sensitivity of Canadian fisheries to climate change is considered moderate and our nations’ capacity to adapt is high relative to less developed countries that are more dependent on fisheries for sustenance. With a changing climate, some harvesting opportunities may be lost while others might be gained by northward movements of species into the LOMAs. This is likely to raise questions concerning the allocation of fishing opportunities among communities.

Establishing and implementing sustainable exploitation rates may be difficult depending on the rates of productivity change in the future. Strategies with more conservative objectives may therefore be required to keep pace with changes in productivity and to build resilience within the exploited population. A key to this resilience is the re-establishment of a diverse age structure among species that were formally much longer lived than they are today and a rebuilding of abundance. Both have known stabilizing effects on population abundance, which on one hand contributes to enhanced interannual predictability of yield and, on the other hand, reduces the risk of collapse or extinction resulting from sporadic mortality events or recruitment failures.

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